1. Field of the Invention
The present invention relates to an image density reproducing method and an image density reproducing apparatus used in a facsimile, an image scanner for processing density data of an image of a document optically read to reproduce the density of the document.
2. Description of the Related Art
In, for example, a facsimile or an image scanner, a document is optically read by a CCD (Charge Coupled Device) image sensor or the like, and an image of the document is stored as digital data. The document image is, for example, printed to be reproduced on the basis of the data. An electrophotographic technique for scanning a photoreceptor by a laser beam to form an electrostatic latent image and developing the electrostatic latent image into a toner image, for example, is applied when printing the image.
If digital data obtained by converting an output of the image sensor is directly binary-coded to form an image, it is impossible to accurately reproduce the density of a halftone document image due to reading characteristics of the image sensor and development characteristics to a toner image.
In order to faithfully reproduce the density of a halftone document image, therefore, data from the image sensor has been conventionally subjected to a variety of processing. Examples of such processing include filtering processing using a so-called Laplacian filter similar filter for compensating for the resolution of an image, gamma correction processing for correcting gamma characteristics of the image sensor, and halftone processing such as a dither method and error diffusion processing.
The gamma correction processing is achieved by preparing a table in which a one-to-one correspondence between input data and gamma correction data is previously established and reading out the gamma correction data corresponding to the input data from this table. Specifically, as shown in, for example, FIG. 15, gamma correction data GOUT are predetermined relative to input data WOUT. A white portion and a black portion of an image are emphasized by this correction. In FIG. 15, the whiter the image is, the larger the value of the data.
Before, for example, the gamma correction processing, when data FOUT corresponding to 16 pixels constituting a matrix with 4.times.4 pixels have values as shown in FIG. 39(a), data GOUT which are subjected to gamma correction shown in FIG. 15 becomes data as shown in FIG. 39(b). In this case, the values of the data FOUT corresponding to the pixels constituting the matrix with 4.times.4 pixels are all "6", and an image area in the matrix with 4.times.4 pixels is an area having a so-called intermediate density.
If an image is binary-coded by performing halftone processing using a dither matrix shown in FIG. 39(c), a binary image printed and outputted becomes an image shown in FIG. 39(d). In FIG. 39(d), a black pixel is indicated by oblique hatching.
As apparent from the comparison between FIG. 39(a) and 39(b), the data corresponding to the 16 pixels are similarly subjected to Gamma correction. Accordingly, in both cases, that is, a case where the data FOUT are directly binary-coded using the dither matrix and a case where the data GOUT are binary-coded using the dither matrix, the image shown in FIG. 39(d) is obtained. Consequently, the same results as those in a case where gamma correction is not made are obtained, so that the density is not sufficiently reproduced.
Specifically, white pixels 161 and 162, for example, surrounded by black pixels are defaced due to the spread of toner particles forming the black pixels, so that the density may be insufficiently reproduced. Consequently, an image portion corresponding to the 4.times.4 pixels may be deep black, thereby making it impossible to reproduce an intermediate density.